1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to an image sensor and fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enabling total photoelectric conversion without light loss by enhancing the surface uniformity of a microlens in each area.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that converts an optical image to an electric signal. And, image sensors are mainly classified into a charge coupled device (CCD) or a complementary metal oxide silicon (CMOS) image sensor.
The image sensor includes a photodiode unit for sensing an applied light and a logic circuit unit for processing the sensed light into data via electric signals. As the quantity of light the photodiode unit receives increases, the better the photosensitivity of the image sensor gets.
To enhance photosensitivity, the fill factor, which is a ratio of a photodiode area relative to the total area of an image sensor, is raised, or a path of incident light on an area other than a photodiode is diverted to be condensed to the photodiode.
A representative example of one condensing technique is the use of a microlens. An increased quantity of light can be applied to a photodiode area by refracting a path of an incident light by providing a convex microlens formed with a substance having good transmittance over a photodiode.
In this case, light parallel to an optical axis of the microlens is refracted by the microlens to have a focus formed on a prescribed position on the optical axis.
Meanwhile, the CMOS image sensor is classified into a 3T type, a 4T type, a 5T type or the like. The 3T type CMOS image sensor includes one photodiode and three transistors. The 4T type CMOS image sensor includes one photodiode and four transistors.
An equivalent circuit and layout of a unit pixel of the 3T type CMOS image sensor are explained as follows.
FIG. 1 is a diagram of an equivalent circuit of a general 3T type CMOS image sensor, and FIG. 2 is a layout of a unit pixel of a general 3T type CMOS image sensor.
Referring to FIG. 1, a unit pixel of a general 3T type CMOS image sensor includes one photodiode PD and three NMOS transistors T1 to T3. A cathode of the photodiode PD is connected to a drain of the first NMOS transistor T1 and a gate of the second NMOS transistor T2. The sources of the first and second NMOS transistors T1 and T2 are connected to a power line supplying a reference voltage VR, and a gate of the first NMOS transistor T1 is connected to a reset line supplying a reset signal RST. A source of the third NMOS transistor T3 is connected to a drain of the second NMOS transistor T2. A drain of the third NMOS transistor T3 is connected to a read circuit (not shown) via a signal line. A gate of the third NMOS transistor T3 is connected to a row select line supplying a select signal SLCT. Hence, the first to third NMOS transistors T1 to T3 are designated reset transistor Rx, drive transistor Dx and select transistor Sx, respectively.
An active area 10, as shown in FIG. 2, is defined in a unit pixel of the general 3T type CMOS image sensor. One photodiode 20 is formed on a wide region of the active area 10 and three gate electrodes 30, 40 and 50 are overlapped with the rest of the active area 10.
In particular, the gate electrode 30 configures a reset transistor Dx. The gate electrode 40 configures a drive transistor Dx. The gate electrode 50 configures a select transistor Sx. The active area 10 of each of the transistors, except the portion overlapped with the corresponding transistor, is doped with impurity ions to become source/drain regions of each of the transistors.
Hence, a power voltage Vdd is applied to the source/drain regions between the reset and drive transistors Rx and Dx, and the source/drain region of the select transistor Sx is connected to a read circuit (not shown).
Moreover, the above-explained gate electrodes 30, 40 and 50 are connected to signal lines (not shown), respectively. A pad is provided to each of the signal lines to connect to an external drive circuit.
An image sensor and method of forming a microlens thereof according to the related art are explained with reference to the drawings as follows.
FIG. 3 is a cross-sectional diagram of an image sensor according to the related art.
Referring to FIG. 3, an image sensor according to the related art includes a sublayer 11 having one or more photodiode areas and metal lines, an insulating interlayer formed on the sublayer 11, an R/G/B color filter layer 13 formed on the insulating interlayer 12 to transmit a light of a specific wavelength, a planarizing layer 14 on the color filter layer 13, and microlenses 15 formed on the planarizing layer 14 overlapped with the color filter layer 13 to have a prescribed convex curvature to condense light.
An optical shield layer (not shown) is provided within the insulating interlayer 12 to prevent light from entering a portion other than the photodiode area.
Alternatively, a photogate can be adopted as a photosensing device instead of a photodiode.
In this case, the color filter layer 13 includes color filters of R (red), G (green) and B (blue). Each of the color filters is formed by coating a corresponding photoresist and by performing exposure and development on the coated photoresist using a separate mask.
The curvature and height of the microlenses 15 are determined by considering various factors such as the focus of condensed light and the like. In particular, the microlenses 15 is formed by coating, patterning, and reflow of photoresist.
Meanwhile, in fabricating a conventional image sensor, since resolution depends on the number of photodiodes existing in the sublayer 11 that receives an image, a unit pixel size is further reduced according to the progress of high pixel implementation and pixel size reduction.
According to the size reduction of the microscopic unit pixel, the input of an external image is condensed to the sublayer using an object lens. The object lens includes the microlens 15.
The color filter layer 13 is classified as a primary color type or a complementary color type. In case of the primary color type, an R/G/B color filter layer is formed. In case of the complementary color type, a cyan/yellow/magenta color filter layer is formed. In this case, the color filer layer 13 is formed on-chip to enable color separation for color reproduction. The color filter layer 13 is formed with an organic substance. After completion of the color filter layer 13, the planarizing layer 14 is formed on the color filter layer 13 for uniformity of the microlenses 15 that will be formed over the color filter layer 13.
In particular, the planarizing layer 14 is hardened by a curing process. The curing process is carried out in a hot plate. The process temperature of the curing is at least 200° C. or above, and the physical property of a surface of the planarizing layer 14 varies according to a solvent component coming from a sealed convection type oven during curing. Hence, flowability of the microlenses 15 that will be formed on the planarizing layer 14 is varied. Thus, if the flowing property of the microlenses 15 is varied, uniformity of the microlenses 15 becomes irregular to cause a light loss.
However, the conventional image sensor and fabricating method thereof have at least the following problem.
After completion of the color filter layer for color separation, the planarizing layer is formed for the uniformity of the surface of the microlenses that will be formed over the color filter layer. In doing so, the planarizing layer is hardened by the curing process. Since the curing process is carried out in the hot plate at the temperature of 200° C. or above, the physical property of the surface of the planarizing layer varies according to the solvent component coming from the sealed convection type oven in curing. Hence, the flowability of the microlenses that will be formed on the planarizing layer is varied. Thus, if the flowing property of the microlenses is varied, uniformity of the microlenses becomes irregular, which causes an unwanted reduction in light.